The present invention generally relates to firefighting equipment, and more specifically, to a hybrid foam proportioning system for fighting fires.
The addition of foaming agents to fire fighting fluids or water streams is well known and can be particularly useful for fighting fires, for example, fires in industrial factories, chemical plants, petrochemical plants, petroleum refineries, forests, and structures. The use of fire fighting foam requires that a foam concentrate be mixed and added at constant proportions to the water stream. When the foam solution is delivered, the foam solution effectively extinguishes the flames of chemical, petroleum, and ordinary combustible fires which would otherwise not be effectively extinguished by the application of water alone.
Foam supply systems known in the art include CAFS (Compressed Air Foam System), WEPS (Water Expansion Pumping System), and EFPS (Electronic Foam Proportioning Systems). A typical foam proportioning system includes a foam injector system and a water pumping system. Whereas a typical CAFS includes a foam injector, a water pumping system, and an air system including an air compressor for supplying air under pressure. For example, when employing mixture ratios of 1/2 to 1 cubic feet per minute (“CFM”) of air to 1 gallon per minute (“GPM”) of water, these systems can produce very desirable results in fire fighting by the use of “Class A” or “Class B” foams to help achieve fire suppression and to deal with increased fire loads and related hazards.
Class A foams are also typically proportioned at 0.1% to 1.0% with an average of 0.4% to 0.5% foam chemical and most often used at flows below 1000 GPM (typically 150-250 GPM). However, Class B foams are proportioned at much higher rates of about 1% to 6% foam chemical typically at about 250 GPM per discharge line for larger hazards. Therefore, for a high flow Class B foam, a much higher foam proportioning capacity is required. However, typical electrical systems on fire apparatus, such as a fire engine, can only support up to a 6 GPM electric pump system. While such systems are suitable for Class A foams, which typically require up to 1.25 GPM of Class A foam concentrate to treat up to 250 GPM of water, such systems are not suited for Class B applications which require about 7.5 GPM or more of foam concentrate to treat about 250 GPM water for a 3% foam chemical. This is where the venturi based, high flow hybrid foam system of the present embodiment advantageously provides the necessary high flow Class B firefighting foam. Class A and Class B relate to fire classes A and B. Class A fires typically involve burning wood whereas Class B fires involve liquid combustible fuels.
Conventional foam proportioning systems typically utilize venturi based proportioning technology. Venturi devices are known proportioning devices creating pressure drops that vary with fluid flow rate in order to proportion foam concentrate into a fire fighting fluid conduit in accordance with varying fire fighting fluid flow rates. Conventional venturi devices accomplish this task with a certain degree of accuracy and efficiency at a fixed flow. In general, the greater the fire fighting fluid flow rate the greater the pressure drop through the venturi, thus drawing in a greater amount of foam concentrate. However, such venturi devices alone are not accurate at low flow rates and their efficiency decreases with high flow rates. The efficiency drops because total pressure drop is in proportion to flow rate and pressure recovery downstream is limited to a maximum efficiency range in the order of 65% to 85% of the pressure drop. Thus, the higher the flow rate, the greater the pressure drop, the less the pressure recovery and the more limited the efficiency. Moreover, conventional venturi devices are not controllable by a user so that such inefficiencies and under or over proportioned foam solutions result due to out-of-control operating conditions of the venturi. Additionally, in a conventional system, the operator has no feedback for adjustment of flow or backpressure which are critical operational parameters for venturi (also know as an eductor) operation. Too much back pressure, for instance will lower or stop foam flow.
The cost of most high volume foam proportioning systems render such systems cost prohibitive for average local fire departments, especially considering that most fires handled by local fire departments are Class A or very small Class B fires, which do not require the assistance of high volume foam proportioning systems. Although smaller foam proportioning systems do exist, such as discharge side pump proportioning systems, such smaller systems do not have the capacity for large Class B fires. As a result, when large Class B fires do arise, under equipped fire departments usually require assistance from other fire departments that may have specialty foam, air port, military, or industrial foam units, or the larger fire burns uncontrolled until enough fuel is consumed that the fire is small enough to be extinguished by the smaller equipment the fire department has in service, obviously creating additional damage and risk. Accordingly, there is a need for a simple, easy to use, controllable foam system that can be readily used for low volume Class A fires and easily converted to a reliable high volume Class B foam flow for Class B fires.